Conventional time-of-flight mass spectrometry (TOFMS) is a technique that uses electron impact (EI) ionization. EI ionization involves irradiating a gas phase molecule of the unknown composition with an electron beam, which displaces outer orbital electrons, thereby producing a net positive charge on the newly formed ion.
TOFMS has seen a resurgence due to the commercial development of two new ionization methods: electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). The availability of low cost pulsed extraction electronics, high speed digital oscilloscopes and ultra-high speed microchannel plate detectors have improved the mass resolution capability of the traditional TOFMS technique.
Mass spectrometers include three major components: (1) an ionization source; (2) a mass filter; and (3) a detector. The ionization source ionizes an unknown composition. The mass filter temporally separates the resultant ions so that lighter ions reach the detector before the heavier ions. The detector converts the ions into a charge pulse. The detector ascertains the arrival times of the charge pulses, which correspond to the masses of the ions. Identifying the masses of the ions enables identification of the unknown composition.
Typically, a TOF mass spectrometer also has a digitizer connected to the detector to process the signals.
In the MALDI technique, the analyte of interest is usually mixed in solution with a large excess of light absorbing matrix material. The sample mixture is placed on a mass spectrometer sample plate and illuminated with a pulse of light from a pulsed laser. The matrix material absorbs the laser light, the analyte molecules are desorbed from the sample surface and ionized by one of a number of ionization mechanisms.
In ESI, the analyte of interest is normally dissolved in an acidified solution. This solution is pumped out the end of a metallic capillary tube held at a high potential. This potential causes the evaporation of extremely small droplets that acquire a high positive charge. Through one of a number of mechanisms, these small droplets continue to evaporate until individual molecular ions are evaporated from the droplet surface into the gas phase. These ions then are extracted through a series of ion optics into the source region of the TOFMS.
The mass filter temporally separates ions by accelerating the ions with a bias voltage ranging up to ±30 kV. Since like charges repel, negative ions, for example, experience repulsive forces, thus tend to accelerate from, a negative potential toward a positive or less negative potential. A higher bias voltage will generate stronger repelling forces, thus greater ion acceleration. The repelling force accelerates lighter particles faster than heavier particles. Although smaller voltages foster better temporal separation, larger voltages allow for greater detection efficiency.
Detectors typically convert an ion into many electrons, forming an electron cloud which is more readily discernable. Three conventional types of detectors, or electron multipliers, generally have been used. The first type of electron multiplier is a single channel electron multiplier (SCEM). SCEMs typically are not used in modem TOFMS instruments because SCEMs provide limited dynamic range and temporal resolution, in the order of 20–30 nanoseconds to full width at half maximum (ns FWHM).
The second type of electron multiplier is a discrete dynode electron multiplier (DDEM). DDEMs exhibit good dynamic range, and are used in moderate and low resolution applications because of relatively poor pulse widths, in the order of 6–10 ns FWHM.
The third type of electron multiplier is a microchannel plate (MCP) electron multiplier. MCPs typically have limited dynamic range, in the order of 20 mHz/cm2 of active area. However, MCPs provide the highest temporal resolution, in the order of 650 ps FWHM.
An ideal TOF electron multiplier should exhibit both high temporal resolution and high sensitivity to high-mass ions, as well as a disinclination to saturation.
As the present invention obtains both high temporal resolution and high sensitivity from an MCP-type electron multiplier, the following reviews the general operating characteristics of an MCP.
FIG. 1 shows an MCP 10. MCP 10 typically is constructed from a fused array of drawn glass tubes filled with a solid, acid-etchable core. Each tube is drawn according to conventional fiber-optic techniques to form single fibers called mono-fibers. A number of these mono-fibers then are stacked in a hexagonal array called a multi. The entire assembly is drawn again to form multi-fibers. The multi-fibers then are stacked to form a boule or billet which is fused together at high temperature. The fused billet is sliced on a wafer saw to the required bias angle, edged to size, then ground and polished to an optical finish, defining a glass wafer 15. Glass wafer 15 is chemically processed to remove the solid core material, leaving a honeycomb structure of millions of pores, also known as holes or channels, 20, which extend at an angle 25 relative to the normal flight trajectory of an ion between the surfaces 30 and 32 of MCP 10.
Referring also to FIG. 2, subsequent processing of the interior surface 35 of each channel 20 produces conductive and secondary electron emissive properties. These secondary electron emissive properties cause channel 20 to produce one or more electrons upon absorption or conversion of a particle, such as an ion, impacting surface 35. As a result, each channel 20 functions like an SCEM, having a continuous dynode source which operates relatively independently of surrounding channels 20.
Finally, a thin metal electrode 40, typically constructed from Inconel or Nichrome, is vacuum deposited on the surfaces 30 and 32 of wafer 15, electrically connecting all channels 20 in parallel. Electrodes 40 permit application of a voltage 45 across MCP 10.
MCP 10 receives ions 50 accelerated thereto by an ion-separating voltage 55. Ion 50 enters an input end 60 of channel 20 and strikes interior surface 35 at a point 62. The impact on surface 35 causes the emission of at least one secondary electron 65. Each secondary electron 65 is accelerated by the electrostatic field created by voltage 45 across channel 20 until electron 65 strikes another point (not shown) on interior surface 35. Assuming secondary electrons 65 have accumulated enough energy from the electrostatic field, each impact releases more secondary electrons 70. This process typically occurs ten to twenty times in channel 20, depending upon the design and use thereof, resulting in a significant signal gain or cascade of output electrons 80. For example, channel 20 may generate 50–500 electrons for each ion.
Gain impacts the sensitivity, or ability to detect an ion, of a spectrometer. A spectrometer with a high gain produces many electrons in an electron cloud corresponding to an ion, thus providing a larger target to detect.
To increase the gain of channel 20, or produce a greater amount of electrons for every ion strike, channel 20 must exhibit enhanced secondary emissivity qualities or conversion efficiency. Enhancing the secondary emissivity qualities of channel 20 is a standing goal.
The gain of channel 20 also is a function of the length-to-diameter ratio (l/d) thereof. This allows for considerable reduction in both length and diameter which permits the fabrication of very small arrays of channels 20 in MCP 10.
In conventional TOF mass spectrometers, electron clouds produced at the channel output are driven toward an anode or charge collector, such as a Faraday cup (not shown). The charge collector sums or integrates the electron charges into a charge pulse, which is analyzed by a digitizer. Because lighter ions accelerate faster than the heavier ions, the voltage pulses correspond to the masses of the respective ions. The aggregate of arrival times of the voltage pulses corresponds to the mass spectrum of the ions. The mass spectrum of the ions aids in discerning the composition of the unknown composition.
Detecting the masses of very massive ions requires a high “post acceleration” potential between the ionization source and the MCP. A high post acceleration potential permits sufficient high mass ion conversion efficiency to enable detection of massive ions. However, MCPs cannot withstand excessive voltages thereacross without risk of significant degradation. Accordingly, some MCP-based spectrometers “float” or electronically isolate the anode from the charge collector. To this end, the MCP output voltage is dropped to ground through a voltage divider. Unfortunately, this creates great potential for arcing or short circuiting between the output and the anode, the energy from which could damage or destroy sensitive and expensive spectrometry equipment. Thus, attaining superior temporal range with an MCP-based spectrometer which also has superior dynamic capabilities, or high sensitivity, may come at significant, unpredictable cost.
Another problem with MCP-based detectors is that, over time, MCPs wear and require replacement. Some mass spectrometers are constructed in a manner that does not permit field replacement of the MCPs. Thus, when an MCP requires replacement, the entire spectrometer had to be returned to the manufacturer for refurbishment. This is undesirable in terms of cost and out-of-service time for the instrument.
To overcome this inconvenience, U.S. Pat. No. 5,770,858 ('858 patent) provides a cartridge containing MCPs which may be installed and uninstalled in the field. However, the charge collector of the '858 cartridge is not electro-optically isolated from the high post acceleration potential of the MCP element therein, like the present cartridge.
Ideally, a TOF electron multiplier should be bipolar, or able to detect both negative and positive ions, which are common to chemical compositions. Thus, the TOF electron multiplier should accommodate positive and negative ion acceleration voltages.
What is needed is a replaceable, electronically-isolated, MCP-based spectrometer detector cartridge with enhanced sensitivity.